An engine of this type, designated in the present field by the terms “open rotor” or “unducted fan” comprises a gas turbine engine feeding a free turbine with contrarotating coaxial rotors each associated with an impeller. The two impellers extend substantially radially to the outside of the nacelle of the turbine engine and are themselves coaxial and contrarotating. The drive of the two impellers is either direct, the two impellers are mounted on the periphery of the two turbine rotors, or by means of a mechanical gearbox, the two impellers each being connected to an output of the gearbox.
Turbine engines with nonstreamlined impellers are currently being studied because they have the advantage of being powerful by being capable of supplying a considerable thrust and of consuming less fuel than other equivalent turbojets with streamlined fan.
However, the high noise levels generated by the mechanisms of aerodynamic interaction between the two impellers are a drawback for this type of propulsion.
One of the sources of this noise arises from the interaction of vortexes generated at the tips of the blades of the upstream impeller, with the blades of the downstream impeller. The vortex generated by the upstream impeller interacts with the downstream impeller extremely energetically which generates high noise levels.
Ways of reducing the impeller noise are aimed at controlling the flows around the profiles; these means are not mature in the current state of the art.
One solution for suppressing this noise consists in reducing the external diameter of the downstream impeller so that the vortices generated by the upstream impeller pass outside the envelope of the downstream impeller and do not interact with the latter. Such a solution is unsatisfactory because it results in a reduction in the thrust produced by the downstream impeller and therefore in a reduction in the performance of the engine. It would be possible to increase the load of the downstream impeller in order to compensate for the reduction in its diameter but this would also increase the aeromechanical difficulty of the design of the pair of impellers and it would become extremely complex and difficult to achieve.